Enhanced optical and electrochemical properties of FeBTC MOF modified TiO2 photoanode for DSSCs application

In this work, iron based 1, 3, 5-tricarboxylic acid (FeBTC) was prepared via microwave-assisted method and incorporated into TiO2 via ultrasonic assisted method. The TiO2–FeBTC nanocomposites were characterized by XRD, FTIR, Raman, BET, FESEM, HRTEM, TGA, UV‒vis DRS and PL to understand their crystallographic, surface morphology, and optical characteristics. The Raman spectra showed a blue shift of Eg, A1g, and B1g peaks upon incorporation of FeBTC MOF onto TiO2. HRTEM and XRD analysis confirmed a mixture of TiO2 nanospheres and hexagonal FeBTC MOF morphologies with high crystallinity. The incorporation of FeBTC onto TiO2 improved the surface area as confirmed by BET results, which resulted in improved absorption in the visible region as a results of reduced bandgap energy from 3.2 to 2.84 eV. The PL results showed a reduced intensity for TiO2–FeBTC (6%) sample, indicating improved separation of electron hole pairs and reduced recombination rate. After fabrication of the TiO2–FeBTC MOF photoanode, the charge transfer kinetics were enhanced at TiO2–FeBTC MOF (6%) with Rp value of 966 Ω, as given by EIS studies. This led to high performance due to low charge resistance. Hence, high power conversion efficiency (PCE) value of 0.538% for TiO2–FeBTC (6%) was achieved, in comparison with other loadings. This was attributed to a relatively high surface area which allowed more charge shuttling and thus better electrical response. Conversely, upon increasing the FeBTC MOF loading to 8%, significant reduction in efficiency (0.478%) was obtained, which was attributed to sluggish charge transfer and fast electron–hole pair recombination rate. The TiO2–FeBTC (6%) may be a good candidate for use in DSSCs as a photoanode materials for improved efficiency.


Synthesis of titanium dioxide (TiO 2 )
The synthesis procedure for TiO 2 is described in our previous report 12 .Typically, 15 mL of titanium isopropoxide and 45 mL of isopropyl alcohol were transferred into a 1000 mL beaker, where 250 mL of distilled water was used as hydrolysis catalyst.After adjusting the pH to 11.5 using NH 4 OH, the mixture was stirred at 500 rpm at room temperature (27 °C) to create a homogenous solution.To catalyse the reaction, the solution mixture was heated for 20 h at 60 °C.The peptization process was followed by reduction in solution to about 30 mL to allow the white viscous suspension to be produced.The obtained white gel solution was washed 3 times using ethanol and then twice with distilled water.After air drying for 24 h, the material was further dried in the oven at 100 °C for 12 h.The TiO 2 powder was obtained by annealing for 2 h at 450 °C in a muffle furnace.

Synthesis of FeBTC MOF
The FeBTC MOF was synthesized via microwave assisted method.In a beaker containing 20 mL of polyethylene glycol (PEG-400), 40 mg of FeCl 3 •6H 2 O was added and allowed to stir for 15 min to create a suspension.In a 20 mL separate beaker, 4.2 mg of trimesic acid was added to 0.01 M sodium hydroxide (NaOH) solution.The two solutions were then mixed and transferred in a glass vial.The glass vial was then sealed with a Teflon lid and put in the microwave reaction vessel.The mixture was allowed to react for 30 min using maximum power of 500 W.After the reaction was complete, the vessel was allowed to cool to room temperature before the resultant suspension was washed with distilled water (3 times) and then with ethanol (2 times) using a centrifuge instrument.The powder was then dried in air for 48 h and thereafter dried for 12 h in an oven at 70 °C.www.nature.com/scientificreports/

Synthesis of TiO 2 -FeBTC nanocomposites
The synthesis of the nanocomposite materials was carried out using ultrasonic assisted method.Firstly, TiO 2 (1.5 g) powder was sonicated for 10 min in a beaker containing 10 mL of ethanol.In a separate beaker containing 10 mL of distilled water, FeBTC MOF (2 mg) was sonicated for 10 min.The two mixture were then transferred into a microwave reaction vessel for 30 min and allowed to cool to room temperature.The resulting suspension was washed in ethanol (3 times) and also in distilled water (2 times) using a centrifuge instrument.The powder was allowed to dry for 48 h in air and then dried in the oven for further 12 h at 70 °C.The powder was then sintered at 300 °C for 45 min.The TiO 2 -FeBTC (2, 4, 6, and 8%) nanocomposites were obtained by varying the loadings of FeBTC MOF.

Characterization techniques
Various instrumental techniques were used to examine the properties of the as-synthesized materials.FTIR spectra PerkinElmer FTIR spectrometer Frontier (spectrum 100 spectrometer) using KBr pellet method at 4 cm −1 resolution was used to study the presence of functional groups.A Witec Raman spectrometer (Alpha 300, TS 150) at 100× magnification and laser power source of 532 nm was used to collect Raman spectra.A Rigaku SmartLab X-ray diffractometer with Cu-Kα (λ = 0.154059 nm) radiation at 2°/min was used to perform XRD analysis.X-ray photoelectron spectroscopy (XPS) analysis was carried out using Thermo ESCalab 250 Xi spectrometer, with Al Kα (1486.7 eV) monochromatic X-ray radiation source which was calibrated by carbon deposit C (1 s) binding energy at 284.8 eV.
A JEOL JSM-7800F field emission scanning electron microscopy (FESEM) coupled with Oxford Aztec 350 X-Max80 electron dispersive spectroscopy (EDS), was used to record surface morphology and elemental analysis.High resolution TEM Tecnai G2F2O X-Twin MAT (Eindhoven, Netherlands) operating at 200 kV was used to analyse the internal morphology of the materials.Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption (Micromeritics Tristar Surface area and Porosity Analyser) was used to obtain the surface area, pore size and volume.To study the thermal stability of the materials under the inert environment from 35 to 900 °C at 10.00 °C/min heating rate a Perkin Elmer Pyris 1 Thermogravimetric analyser (TGA) was used.The optical properties were evaluated using PerkinElmer UV/Vis/NIR spectrometer Lambda 1050 from 200 to 800 nm wavelength range.A Horiba Fluorolog-3 Jobnyvon at the excitation wavelength of 325 nm was used to record photoluminescence spectra.

Fabrication of the photoanodes
Firstly, the FTO glass substrates (13.7 Ω) were soaked in a beaker containing soap and ultrasonicated for 15 min and rinsed with distilled water.They were further ultrasonicated in absolute ethanol for 15 min and rinsed with distilled water.The substrate was then baked in the furnace at 500 °C to remove any organic residues and thereafter stored in a clean desiccator to prevent any further contamination.The paste was obtained by dissolving 2 g of powder in 1.5 mL of dimethyl sulfoxide (DMSO) and allowed to sonicate for 10 min.The resultant paste was dropped on a clean FTO substrate and spin coated at 6000 rpm for 80 s.The coated films were then heated on the stove for 30 min at 90 °C, after which the films were baked in the furnace at 450 °C for 30 min (Fig. 1).

Photoelectrochemical studies
A computer-controlled potentiostat/galvanostat (galvanostat (Autolab, PGSTAT 302 N model, Metrohm, Swiss instruments) was used to obtained electrochemical data for the photoelectrodes.A three-set-up electrode, platinum foil, Ag/AgCl and prepared photoanodes (1.4 × 1.4 cm) were used as counter, reference and working electrodes respectively.An electrolyte containing 5 mM acetonitrile solution, 0.055 M LiI, 9 mM I 2 and 0.85 M LiClO 4 was used to carry the measurements.All EIS measurements were carried out at the frequency range of 1-100 kHz with voltage amplitude of 25 mV under the dark.To obtain photodensity measurements, a solar simulator equipped with a Xenon lamp and AM 1.5 filter under one-sun illumination (100 mW/cm 2 ) was used.

XRD analysis
To evaluate the crystallographic information, XRD analysis of as-synthesized samples was performed as presented in Fig. 2. The results showed that, TiO 2 exhibits indexed diffraction peaks of a tetragonal anatase phase (peaks at 2Ɵ = 25.58°(101), 37.8.17° (004), 48.4° (200), 55.33° (105), 61.20° (204), 68.5° (220) and 76.1° (301) corresponding to the standard data, card no.01-075-2552.These peaks were also observed in TiO 2 -FeBTC (2, 4, 6 and 8%) nanocomposite materials.However, there was a red shift with reduced peak intensity in all of the nanocomposite materials, which highlights the interaction between TiO 2 and FeBTC.As it is reported elsewhere, the shifting of the peaks is related to structural relaxation upon the introduction of Fe-MOF on to TiO 2 and this is due to changes in lattice parameters 12,32 .
The crystallite size, micro-strain and the D spacing results are given in Table 1.The crystallite size and microstrain were estimated using the Eqs.where θ is the glancing Braggs angle with the sample holder, λ = 0.15418 is the wavelength of CuKα radiation, and β is the line broadening at half the maximum intensity (FWHM).Accordingly, crystallite size ranged between 9.7-16 nm for the as-synthesized materials.There was a decrease in crystallite size and micro strain with FeBTC loading suggesting a change in crystallinity of TiO 2 upon the addition of FeBTC MOF.
To validate the chemical composition and oxidation state of the elements, XPS analysis was carried out as shown in Fig. 2b-d.The wide spectra in Fig. 2b showed the elemental constituents of pure TiO 2 and TiO 2 -FeBTC (6%) material.In Fig. 2c the high resolution spectra of TiO 2 showed the presence of two peaks at 463.8 and 458.1 eV, representing the Ti 2p 1/2 and Ti 2p 3/2 of the core level of Ti 4+ , respectively.In TiO 2 -FeBTC (6%), these peaks are located at 464.1 and 458.3 eV for Ti 2p 1/2 and Ti 2p 3/2 .Additionally, the Fe 2p spectra in Fig. 2d showed two main peaks at 724 and 732 eV which are assigned to Fe 2p 3/2 and Fe 2p 1/2 in Fe-BTC MOF, respectively.These two peaks with the presence of satellite peak at 729 eV suggest that the Fe-BTC MOF exist in the trivalent (Fe(III)) state.The O1s spectra in TiO 2 -FeBTC (6%), demonstrated a slight shift in binding energy, indicating a formation of C-Ti-O bonding (Fig. S1).The shift in binding energy suggest a strong interaction between the FeBTC and TiO 2 , which might facilitate the electron transfer between the material upon light illumination and applied potential/current density.

Morphological properties
SEM analyses of nanocomposite materials SEM images of the as-synthesized materials are shown in Fig. 3.The SEM image of TiO 2 in Fig. 3a shows small aggregated spheres of nanoparticles.The aggregation may be due to stirring during the gelation in the synthesis step.The FeBTC MOF in Fig. 3b is characterized by irregular particles of different shape and size.The SEM images of TiO 2 -FeBTC MOF nanocomposites (Fig. 3c-f) are characterized by TiO 2 nanoparticles dispersed on the surface of larger FeBTC MOF nanoparticles.An increase in aggregation of nanoparticles, with an increase in FeBTC MOF loading was observed.The increase in aggregation may reduce the surface area of the materials and therefore hinder electron-charge transfer as there will be limited surface area for electron transfer reactions.Furthermore, EDS spectra in Fig. 3g showed the expected elements of TiO 2 (Ti, O, and C) and Fe-BTC (Fe, C, and O) in the nanocomposites.The EDS of TiO 2 -FeBTC (2, 4, 6, and 8%) are characterized by dominant Ti, O and C elements.Fe element was not observed in the nanocomposites materials and this could be due to low concentration used during the synthesis step.

HRTEM analysis
The morphology of the as-synthesized materials was further evaluated by HRTEM.Image J software was used to estimate the particle size of the materials.Figure 4a shows HRTEM image of TiO 2 which is characterized by aggregated nano-spheres with varying particle sizes, ranging between 4-35 nm.The variation of size was induced www.nature.com/scientificreports/by the stirring during the gelation process 12 .The FeBTC MOF exhibited hexagonal shapes with poorly formed aggregated nanoparticles of different sizes as seen in Fig. 4b.The nanoparticles have a characteristic of moderate sorting which implies that the particle size variation is small.The particle size of FeBTC MOF ranged between 20-80 nm.The HTREM images for TiO 2 -FeBTC(2,4,6 and 8%) nanocomposites are shown in Fig. 4c-f respectively.The HRTEM images of the nanocomposites are characterized by a mixture of smaller TiO 2 nanoparticles and much larger FeBTC MOF particles, attached to surface of each other.The FeBTC MOF nanoparticles are well dispersed on the surface of TiO 2 and this can be seen in all nanocomposites.The increase in loading of FeBTC MOF is also evident as the number of larger particles are observed at higher loadings from Fig. Fig. 4c-f.Hence, the aggregation of particles is more prevalent in TiO 2 -FeBTC (8%) nanocomposite (Fig. 4f) and thus, leading to a reduced pore sizes.The reduction of pore volume and sizes is an undesired consequence as it can reduce the charge transportation of electrons and also impact electron collection by the photoanode half of DSSCs, which will result in higher recombination rate of electron-hole pairs.

adsorption-desorption isotherms
The BET results in Table 2 show that TiO 2 exhibited a low specific surface area of 106.75 m 2 /g, which is consistent with our previous findings 12 .However, upon incorporation of FeBTC MOF, the surface area of 636.53 m 2 /g, which was four times higher than that of TiO 2 was obtained.This means that, incorporation of MOF on TiO 2 matrix, considerably improves the specific surface areas.Consequently, he surface area was further improved with an increase of MOF loading by 2, 4, 6 and 8% reaching 188.5 m 2 /g, 256.71 m 2 /g, 328.54 m 2 /g, and 336.21 m 2 /g   www.nature.com/scientificreports/respectively (Table 2).Similarly, the pore size and pore volume of the TiO 2 -FeBTC nanocomposites also increased with an increase in MOF loading from 2 to 6%.However, the TiO 2 -FeBTC (8%) nanocomposite showed a decrease in pore volume and pore size, which is attributed to higher aggregation of the Fe-BTC MOF and TiO 2 nanoparticles blocking the pores as evident from the SEM and HRTEM results.Generally, DSSCs rely on the photoanode to absorb light and also generate electrons and therefore, the high surface area is an important property as it will aid in adsorption of the dye molecule on the surface of TiO 2 -FeBTC MOF and thereby improving sunlight absorption.The N 2 adsorption-desorption isotherms corresponding to these samples are presented in Fig. 5a-f.The pure TiO 2 in Fig. 5b and the nanocomposite materials in Fig. 5c-f, showed a type IV isotherm which suggest that there are stronger lateral interactions between adsorbed molecules.The hysteresis loop showed by the nitrogen adsorption-desorption isotherm of TiO 2 confirms that the materials are mesoporous and resemble that of type H3 IUPAC (International Union of Pure and Applied Chemistry) classification.The FeBTC MOF isotherm in Fig. 5a, resembles that of type H1 which correlates with the porous material 33 .

Functional and thermal properties
Raman analysis.The Raman spectra of pure TiO 2 and TiO 2 -MOF nanocomposites (2, 4, 6 and 8%) is shown in Fig. 6a.The Raman spectrum of TiO 2 , demonstrated distinct Raman active modes of the E g (142.7 and 635.6 cm −1 ), B 1g (396.5 cm −1 ) and A 1g (514.1 cm −1 ) which are characteristics of the anatase phase as also confirmed by XRD results.The Raman modes of anatase phase (E g , B 1g and A 1g ) are due to symmetric stretching vibration, symmetric bending vibration and the asymmetric bending vibration of O-Ti-O, respectively 34 .The E g peak of anatase at 142.7 cm −1 decrease in intensity with an increase in Fe-BTC MOF loading, and there is blue-shift observed for all the E g , B 1g and A 1g of Raman modes in the TiO 2 -FeBTC (2, 4, 6 and 8%) nanocomposites.The increase in intensity is related to increase in crystallinity during the formation of the TiO 2 -FeBTC nanocomposites due to three-dimensional phonon confinement 35 .The shifting of Raman modes to lower wavenumber is related to an increase in crystal size of the TiO 2 -FeBTC (2, 4, 6 and 8%) nanocomposite materials 36 .In addition, the shifting in Raman peaks may be related to migration of electrons from the TiO 2 nanoparticles which decrease the band strength within the O-Ti-O bond 12,37 .The change in Raman modes in all the nanocomposites spectra suggests that there is an interaction between the TiO 2 and FeBTC MOF. of TiO 2 and FeBTC MOF materials.It is worth noting that there is a decrease in intensity in TiO 2 -FeBTC (8%) nanocomposite due to increase in FeBTC concentration.TGA analysis.To investigate the thermal stability of the as-synthesized materials, thermal gravimetric analysis (TGA) was performed as shown in Fig. 6c.The weight curve for Fe-BTC MOF shows several decomposition steps.At 100 °C, there is mass loss due to loss of moisture from water molecule.There is a mass loss due to destruction of the organic ligands was observed between 100 and 380 °C, while a total destruction of the Fe-BTC occurred from 400 to 550 °C, leaving only iron as the stable species.A minimal loss due to moisture below 100 °C was observed for TiO 2 , however, it remains stable throughout the temperature.The TiO 2 -FeBTC nanocomposites show relative stable curves compared to pure FeBTC MOF.However, as the Fe-BTC MOF loading is increased, the TiO 2 becomes less thermally stable as evident by TiO 2 -FeBTC (2, 4, 6, and 8%) nanocomposites.This is induced by the less stable organic linker present in FeBTC MOF.

Optical and electrochemical properties
UV-Vis spectroscopic analysis Figure 7a, shows the UV-Vis diffuse reflectance spectra of pure TiO 2 , TiO 2 -Fe-BTC MOF (2, 4, 6, and 8%) nanocomposite materials.The pure TiO 2 absorbs only in the UV region, with the absorption edge around 400 nm.The TiO 2 -FeBTC nanocomposites spectra are characterized by two absorption peaks with different intensity at 422 nm and around 550 nm.The intensity of these peaks increases with an increase in FeBTC MOF loading, with the TiO 2 -FeBTC (8%) showing the highest intensities.The appearance of the peak in the visible region suggest that there is an improvement in band gap energy which will allow TiO 2 to absorb in this region.The Tauc plots in Fig. 7b confirmed that as the FeBTC MOF loading is increased, the bandgap energy narrows, which allows efficient charge transfer of electro-hole pairs and hence reduce recombination rate.This was expected as the FeBTC MOF has an intense broad peak in the visible region of 400-620 nm.This is desirable in the fabrication of DSSCs because it allows photoanode to absorb more light and improve the electron shuttling in the conduction band of TiO 2 , thereby improving the overall efficiency in the process.

Photoluminescence analysis
Photoluminescence (PL) was analysed to gain deeper insights into the photogenerated charge transport process in photocatalytic reaction by investigation of the efficiency of charge carrier trapping, transfer and migration and also understand the fate of electron-hole pairs in the as-synthesized nanocomposites.The recombination of the excited electrons and holes induce the PL emission.Under light irradiation, PL spectra was reported from excited electrons and holes, where the highest intensity correspond to higher recombination rate of electronshole pairs 39,40 .The excitation wavelength was at 325 nm for all the as-synthesized materials under this study.In Fig. 8a, pristine TiO 2 has the higher intensity, which suggest that the material has the fastest recombination rate of holes and electrons relative to the FeBTC MOF and the nanocomposite materials.This indicate that the addition of FeBTC MOF reduced the recombination rate by bandgap engineering as suggested by UV-Vis studies.The addition of the MOF reduces the recombination rate of photogenerated charge carriers of electron-hole pairs.The 8% loading demonstrated a decrease in intensity which suggest an increase in recombination rate due to aggregation of FeBTC nanoparticles (Table 2).The increase in recombination rate may be related to the development of trapping sites which affects electron trapping in the shuttle process and therefore their development may greatly affect the performance of our materials in DSSCs.

EIS analyses for the TiO 2 , and TiO 2 -FeBTC MOF photoanodes
To further investigate the electron-hole pair separation behaviour of our materials, impedance studies were conducted on the prepared photoelectrodes.The photoanodes exhibited varied thickness of 17.26, 16.14, 14.11,However, there is a sudden increase in charge transfer resistance when the loading is increased to 8%, which may be related to aggregation of TiO 2 -FeBTC nanoparticles, and thereby reducing the surface area and the pore size, which are the charge pathways and thus charge separation is compromised.This is supported by the SEM and BET results that showed that there is a decrease surface area due to increased aggregation.It was noted that the thickness of the photoanodes did not have correlation with the electron transportation.The decrease in charge transfer resistance with FeBTC MOF (6%) loading is due to the improved electron shuttling away from the hole in the TiO 2 surface and thus retarding recombination rate of electron-hole pairs.

Photoelectric performance of the TiO 2 -FeBTC MOF photoanodes
Photoelectric performance of the prepared thin film photoelectrodes are presented in Fig. 9a.Accordingly, under dark, the current was very low and therefore negligible.However, upon illumination, there was an increase in magnitude of current and again an increase in current with FeBTC MOF loading of the photoelectrode.The highest current was observed in the TiO 2 -FeBTC (6%) and was five times higher than that of pure TiO 2 .It is worth noting that beyond 8% loading of FeBTC MOF, there is a decrease in current generated.The charge resistance also starts to build up which inevitably increases the recombination rate.Power conversion efficiency as shown in Fig. 9b, was calculated from the I-V curves in Fig. 9a using the following equation: where, Jp is the photocurrent density (Am/cm 2 ), E 0 rev is the standard state-reversible potential (1.43 vs Ag/ AgCl) of the working electrode, E cou is the open-circuit potential (vs Ag/AgCl) under the same working electrode experimental conditions, I 0 is the power density of the incident light and E mea is the electrode potential of the working electrode whose photocurrent was tested under irradiation.
As expected from the I-V curves, the loading of FeBTC MOF increased the photoefficiency of the photoelectrodes.The photoefficiency values of TiO 2 , TiO 2 -FeBTC (2, 4, 6 and 8%) nanocomposites were found to be 0.145, 0.298, 0.421, 0.562, and 0.497%, respectively.The TiO 2 -FeBTC (6%) loading, exhibited high photo conversion efficiency.In comparison with pure TiO 2 photoanode, the photoefficiency of TiO 2 -FeBTC (6%) was over five times.These results are consistent with what we observed in the PL and EIS analysis.This suggest that TiO 2 -FeBTC (6%) can be a suitable candidate to use as a photoanode material in the development of DSSCs.To validate this work, the results were also compared to other reported TiO 2 photoelectrodes as shown in Table 3.The TiO 2 -FeBTC (6%) photoefficiency was comparable with other developed electrodes and therefore can be suitable for use in DSSCs.

Conclusions
This work reports the synthesis of TiO 2 -FeBTC nanocomposites through ultrasonic-assisted method and their subsequent fabrication into photoelectrode via spin coating technique.The surface, structural properties and optical properties were evaluated by XRD, FTIR, Raman, BET, TGA, FESEM, TEM, UV-Vis DRS, and PL techniques.Enhanced surface area and high pore volumes were observed after incorporation of FeBTC MOF onto TiO 2 , which also improved charge transfer kinetics and reduced electron-hole recombination rates as confirmed by EIS and PL, respectively.The TiO 2 -FeBTC (6%) showed good charge separation of electron-hole pairs due to low charge resistance.The improved charge transport due to surface area has allowed the 6% loading to have excellence photoelectrical response as confirmed by I-V curves.The I-V results have showed that there is a direct proportional relationship between the generated photocurrent and FeBTC MOF loading.The optimal photocurrent density was observed in the TiO 2 -FeBTC (6%) at 0.87 mA/cm 2 .This resulted in high PCE value of 0.538% of TiO 2 -FeBTC (6%).This was attributed of relatively high surface area, which allowed more charge shuttling and thus better electrical response.Further increase in loading above 6% resulted in higher aggregation of TiO 2 which gave rise to poor electron transport and thus higher recombination rate build-up.Therefore, it is necessary to optimise the loading of FeBTC MOF, as this can affect the performance of the photoanode in DSSCs.Based on the assessed performance of TiO 2 -FeBTC (6%), can form a viable candidate as a photoanode materials for DSSCs. https://doi.org/10.1038/s41598-024-61701-3 https://doi.org/10.1038/s41598-024-61701-3

Figure 6 .
Figure 6.Shows the (a) FTIR, the (b) Raman spectra and (c) thermal gravimetric analysis of the as-synthesized materials.

Figure 8 .
Figure 8.The EIS curves (a) and PL spectra (b) of the fabricated electrodes and (c) the Randel circuit used to fit EIS curves.

Figure 9 .
Figure 9. (a) I-V curves of TiO 2 (a) and TiO 2 -FeBTC (2, 4, 6 and 8%)/thin film photoelectrodes and (b) the corresponding photocurrent conversion efficiency.The experiment was performed in a solution with 5 mM acetonitrile solution, 0.055 M LiI, 9 mM I 2 and 0.85 M LiClO 4 in the dark and under illumination using one-sun illumination (100 mW/cm 2 ) and (b) shows the corresponding photo-conversion efficiency.

Table 1 .
Crystallite size, micro-strain and D-spacing of the as-synthesized materials.

Table 2 .
Band gap energy, BET surface area, pore size, pore volume and particle size of as-synthesized materials.